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What is the difference between a selective ferrous/non-ferrous sensor and (factor 1) ferrous/non-ferrous sensors?

Ferrous (Fe) and non-ferrous (Nfe) terminology relates to inductive proximity sensors.
Inductive proximity sensors work by inducing a current in a metal object that is within the operating range of the sensor. The inductive sensor generates a magnetic field by use of an oscillator. When a metal object is close enough to be within the magnetic field, the sensor electronics detect a load change within the oscillator and the sensors signals this via the output circuit.

Several factors affect the sensing range of an inductive proximity sensor. Because of this, standard  inductive proximity sensors are given a 'nominal sensing distance' (Sn) that is based on the theoretical maximum distance the sensor can detect a metal object. In practice, because of the size of the object, variation in temperature, applied voltage and type of metal, the actual sensing distance achieved is invariably less than the Sn.

A key factor in the sensing range achieved is the type of metal to be detected. Note: Inductive sensors can only detect metal.
Of particular importance is whether the metal is ferrous or non-ferrous. 
A metal that contains an appreciable amount of iron is considered to be ferrous, these include; cast iron, steel and steel alloys. As the amount of iron in the alloy reduces, so the sensing distance that a standard inductive sensor can achieve reduces.
Non-ferrous metals, such as brass and copper do not contain an appreciable amount of iron and as a result detection performance of a standard inductive sensor is considerably reduced compared to Sn.

Refer to the graph below that indicates the approximate effect of the type of metal on sensing performance of a standard inductive sensor, e.g. steel can be seen to be a coefficient (km) factor of 1 (so no effect on sensing performance), whereas copper is a factor of 0.25 so the sensing distance will only be a quarter of what it would be with steel; 


Factor 1 ferrous/non-ferrous sensors

The effect on sensing distance can be problematical, for instance if both steel and aluminum cans need to be detected at the same distance from a sensor. For this reason (Factor 1) 'ferrous/non-ferrous sensors' are available from Schneider Electric.
The sensing distance for these sensors is the same for both ferrous and non-ferrous materials. The Km coefficient is always 1.
Principle of operation
These sensors use a very high frequency oscillator (in the region of MHz) that performs the same with both ferrous and non-ferrous metals.

Schneider Electric part numbers;

  • XS1M18KPM40 diameter 18mm (version with connector also available)
  • XS1M30KPM40 diameter 30mm (version with connector also available)
  • XS7C40KPM40 Form C

Selective ferrous and selective non-ferrous sensors

Principle of operation
These sensors use the effect on attenuation of oscillation amplitude according to the metal to differentiate ferrous and non-ferrous, attenuation is significant for ferrous materials and insignificant for non-ferrous metals.  It is therefore possible to design the sensor to detect only  ferrous or only non-ferrous metals;

Schneider Electric part numbers;

  • XS1M18PAS40 (version with connector also available)  ferrous version (insensitive to non-ferrous materials)
  • XS1M18PAS20 (version with connector also available)  non-ferrous version (insensitive to ferrous materials)
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